This invention relates to the production of foam particles suitable for secondary foaming. More particularly, the invention relates to a process and apparatus for producing gas pressurized foam particles.
Thermoplastic resin foam materials with closed-cell structures are well known. Such materials are made by incorporating a physical blowing agent or a chemical blowing agent into a resin composition and thereafter heating the composition. Gases which are created during the heating step expand the resin material to form a cellular mass. It is also known that the resultant closed-cell foam material can be induced to further expand. This added expansion is referred to as secondary foaming. Thus, a thermoplastic closed cell foam material is initially subjected to an inert gas at superatmosphere pressure. The inert gas will ultimately permeate into the foam material's closed cells. Exposure of the pressurized foam material to a lower pressure will cause internal extensional stress. This stress causes the foam to expand further to create an even lower density material when the polymer matrix is heat softened sufficiently to stretch under the influence of this stress. See, for example, Rubens Pat. No. 4,360,484, assigned to the same assignee as the present invention, for a more complete description of this general process and a discussion of the low temperature storage of pressurized prefoamed materials feature.
Closed cell thermoplastic foam materials which have been pressurized for a secondary foaming step are often used in the making of molded articles. Several patents, U.S. Pat. Nos. 2,898,632 and 3,953,558 in particular, describe processes of making molded articles from pressurized foam beads or pellets. See also Rubens et al Pat. No. 4,693,856, assigned to the same assignee as the present invention, for discussion of a molding method and apparatus which may utilize secondary foaming.
A problem which has slowed the full commercialization of the secondary foaming technology has been the inability to efficiently produce gas pressurized closed-cell foam particles suitable for further processing. The processing time needed for an inert gas to permeate into the closed cells of a foam particle is inordinately long, e.g. up to thirty hours even at high pressures. Bulky space consuming processing equipment is also needed. This is in part due to a need for the foam particles to receive a uniform pressurization time for consistent secondary foaming. It is also due in part to a need to ensure that the foam particles are not constricted within the processing equipment as they increase in volume. That is, as the particles expand they tend to agglomerate and bridge across openings, making handling difficult.
Accordingly, the need exists for an improved process and apparatus which allows for the efficient production of the foam particles by substantially reducing process time and equipment needs.